When evaluating the composition of a substance, it is often desirable to study the behavior of charged particles (e.g., ions, electrons) generated from a sample of the substance of interest. Charged particles are often discharged from the sample in the form of atomic or molecular ions; however, in some cases it may be desirable to study subatomic or larger (i.e., nanomaterial) particles bearing a charge.
Various types of instruments have been developed to facilitate the evaluation of charged particles, including, for example, ion mobility spectrometers, time of flight mass spectrometers, multi-pole mass spectrometers, and cyclotrons. Such instruments may be commonly used to detect explosives, narcotics, and chemical warfare (e.g., nerve and blister) agents. The instruments used to facilitate evaluation of the charged particle generally include the controlled generation of one or more electric fields. For example, some instruments may utilize electric fields to accelerate, separate, and otherwise selectively direct charged particles. Expanded application of charged particle analysis often entails the careful design of an electric field tailored with various predetermined characteristics. Examples of such predetermined characteristics may include the shape or focal points (i.e., the desired destination of the charged particles) of the electric field and the spatial orientation of the electric field relative to the desired pathway of charged particles within the instruments.
There are different approaches in generating electric fields for directing charged particles. One approach for an electric field generator is to apply different voltages to a plurality of conductive parts (sometimes called “lenses” or “conductive electrodes”) spaced apart from each other. If a voltage is applied to the conductive electrodes, an electric field is generated. The voltages applied to a plurality of conductive electrodes combine to form the electric field. Certain complex electric fields (e.g., quadrupolar) have been generated by the arrangement of the plurality of conductive electrodes. Another approach for generating an electric field may include applying a voltage to an electrode formed from a semiconductive material, which semiconductive material is a simple shape (e.g., formed as a simple tube or plate) and uniform in resistivity in order to generate a simple (e.g., linear) electric field.
One factor that may influence properties of the electric field includes the orientation of the electrodes relative to each other. Conventionally, the resulting complex electric fields or simple electric fields are determined by, and substantially emulate (i.e., mirror) the shapes and configurations of the electrodes used to generate the electric field. For example, a linear electric field is generated conventionally using a cylindrical electrode or a rectangular bar (e.g., a linear drift tube). A quadrupolar electric field is generated conventionally using conductive electrodes configured in a physical shape that is substantially similar to the shape of the electric field (e.g., an ion trap). This conforming of the physical configuration of the electric field generator and the resulting electric field may reduce the flexibility of the shapes that are to be used to generate a given electric field, and therefore, may limit the shape and configuration of the device that may include the electric field generator.
FIG. 1A is a schematic depicting a conventional electric field generator 100. For example, the electric field generator 100 may be implemented as an interface for confining and releasing charged particles within an ion mobility spectrometer (not shown). The electric field generator 100 may include a plurality of conductive electrodes 110, 120, 125, 130, 135 and a voltage source 140. The conductive electrodes 110, 120, 125, 130, 135 may be fabricated from electrically conductive materials (e.g., metals and metal alloys). Specifically, the electric field generator 100 includes an outer electrode 110, and inner electrodes 120, 125 within the interior 105 of the electric field generator 100. The electric field generator 100 further includes end cap electrodes 130, 135. As shown in FIG. 1A, each end cap electrode 130, 135 may include a portion extending into the interior 105 of the electric field generator 100.
In operation, a voltage source 140 may be connected to the different conductive electrodes 110, 120, 125, 130, 135, such that an electric field is generated when a voltage is applied to one or more of the conductive electrodes 110, 120, 125, 130, 135 (see FIG. 1B). The voltage source 140 may provide voltages of the same voltage potential or voltages of a different voltage potential to each of the conductive electrodes 110, 120, 125, 130, 135, as the case may be. For example, the voltage source 140 may include a resistive ladder (not shown) configured to generate voltages 141-145 at different nodes between the individual resistors of the resistive ladder. Resistive ladders may require careful selection of components, each of which may fail separately or characteristics of which may change with temperature, which may lead to distortion of a desired electric field generated by the conductive electrodes 110, 120, 125, 130, 135. Alternatively, the voltage source 140 may include control logic to independently control the voltage level of voltages 141-145 according to voltage functions, which may control or alter the shape of the electric field depending on the relative strength of each voltage level of voltages 141-145.
FIG. 1B depicts a resulting electric field 150, which may be generated by the conventional electric field generator 100 of FIG. 1A. The electric field 150, as shown, may be a quadrupolar shape, which may be useful for confining charged particles in a given space. Conventionally, the quadrupolar shape of the electric field 150 substantially emulates (i.e., mirrors) the physical configuration of the electrodes 110, 120, 125, 130, 135 of FIG. 1A. Conventionally, quadrupolar electric fields may be generated by configuring the conductive electrodes of an electric field generator in hyperbolic shapes. For example, when the voltage source 140 applies a voltage to each of the electrodes 110, 120, 125, 130, 135, portions of the resulting electric field 150 can be seen to mimic the shape of the physical configuration as shown by the numerical indicators of FIG. 1B.
FIG. 2A is a cross-sectional view of a schematic of a conventional electric field generator 200. The conventional electric field generator 200 may be configured as a cylinder with an inner diameter 201 through which charged particles 206-208 may travel. The generated electric field 205 may be configured to direct the charged particles 206-208 through the cylinder. The conventional electric field generator 200 may include a plurality of conductive electrodes 210-216. Because each conductive electrode 210-216 may be at a different voltage potential, conductive electrodes 210-216 may need to be electrically isolated from each other. Regions 220-225 may provide the electrical isolation for conductive electrodes 210-216. Regions 220-225 may be voids (i.e., air), or may be insulators. One problem encountered with having a plurality of conductors 210-216 separated by regions 220-225 is that the interface between a conductive electrode (e.g., 210) and an adjacent region (e.g., 220) may create a ridge. Such a ridge may alter airflow across the surface of the conventional electric field generator 200. Such a turbulent airflow may reduce the effectiveness of the conventional electric field generator 200 in directing the charged particles 206-208. Additionally, when regions 220-225 include insulators that may be exposed to the charged particles 206-208, the insulators themselves may become charged, which may distort the electric field 205.
FIG. 2A also shows that the electric field 205 generated by conventional electric field generators 200, employing stacked conductive electrodes 210-216 and regions 220-225, may have nonlinear portions near the conductive electrodes 210-216 that may cause the charged particles 207, 208 located away from the center of the cylinder to drift toward the conductive electrodes 210-216. These nonlinear portions of the electric field 205 may cause the charged particles 207, 208 located off-center to contact the conductive electrodes 210-216 or to have an undesirably different path length in contrast with the charged particles 206 located near the center of the cylinder. This undesirably different path length may cause the charged particles 207, 208 to arrive at a desired location at a different time than charged particles 206.
FIG. 2B is a graph showing boundary voltages 230 along a conventional electric field generator such as, for example, the conventional electric field generator 200 of FIG. 2A. The boundary voltages 230 may generate an electric field 205, which, as shown by FIG. 2B may be pseudo-linear. For example, the conventional electric field generator 200 may experience a voltage drop between VH and VL. For example, voltages V6-VL may be applied to conductive electrodes 210-216, respectively. Conductive electrodes 210-216 may create discontinuities (i.e., gaps) in the resulting boundary voltages 230, which may distort the boundary voltages 230 from generating the desired electric field 205. For example, in an alternating stack of conductive electrodes 210-216 and regions 220-225 (e.g., insulators) therebetween, the boundary voltages 230 change across the regions 220-225, while the boundary voltages 230 are substantially flat across the conductive electrodes 210-216 because the voltage across a conductor is essentially constant. These discontinuities or “steps” may not be the desired effect for the electric field 205, yet the steps may not be avoided with conventional conductive electrodes 210-216. Because these discontinuities are more apparent near the surface of the conventional electric field generator 200, the discontinuities may be more exaggerated in applications that have a relatively small scale.
In general, the conductive electrodes 210-216 may cause the conventional electric field generator 200 to be relatively complicated to construct, as multiple conductive parts must be precisely positioned in relation to each other in order to obtain the desired electric field 205. In addition, electrodes 210-216 separated by insulators may be relatively heavy, which can be an issue for miniaturization or for aerospace applications.
FIG. 2C is a perspective view of a conventional electric field generator 250. The conventional electric field generator 250 may be configured as a generally elongate, cylindrically shaped member having a first end 255, a second end 260, and a length 270. The conventional electric field generator 250 may be formed from an electrically semiconductive material so that a voltage potential may be established along the axis of conventional electric field generator 250.
In operation, a voltage source (not shown) may provide voltages of different potentials to the first end 255 and the second end 260 of the conventional electric field generator 250, causing a voltage drop across the conventional electric field generator 250. A voltage drop across the conventional electric field generator 250 results in the generation of an electric field within an interior region of the conventional electric field generator 250.
FIG. 2D depicts a resulting electric field 275, which may be generated by the conventional electric field generator 250 of FIG. 2C. Conventionally, the resulting electric field 275 substantially emulates (i.e., mirrors) the physical shapes and configurations of the electrodes used to generate the electric field. For example, a linear electric field 275 is conventionally generated by a cylindrical electrode, such as with the generally elongate, cylindrically shaped member of the electric field generator 250 of FIG. 2C.
FIG. 3 is a schematic of a conventional charged particle guide 300. Charged particle guide 300 may approximate an inlet to a mass spectrometer, such as the LCQ FLEET™ Ion Trap, available from Thermo Fisher Scientific, Inc. of Waltham, Mass. The electric field 305 and direction of charged particles 306 shown in FIG. 3 is modeled with a statistical diffusion simulation (SDS) in the simulation software, SIMION®, which software is available from Scientific Instrument Services, Inc. of Ringoes, N.J. Other simulations shown herein may also be modeled in SIMION®.
Conventional charged particle guide 300 may be configured to direct charged particles 306 toward an aperture 335 defined by a structure 330, after which the charged particles 306 may be further analyzed, re-directed, or otherwise processed, as desired. For example, a conventional charged particle guide 300 may be implemented as part of a conduit, which may assist the transfer of charged particles 306 generated at a high-pressure region (e.g., atmospheric region) into a low-pressure region (e.g., vacuum region) of an instrument, such as a mass spectrometer.
Conventional charged particle guide 300 includes conductive electrode 310 configured for generating an electric field 305 when a voltage is applied to the conductive electrode 310 by a voltage source (not shown). The voltage of the conductive electrode 310 and the structure 330 may be substantially equal. As with other conventional electric field generators, the physical shape of the configuration of the conductive electrode 310 substantially emulates the shape of the electric field 305 generated by the conventional charged particle guide 300. The electric field 305 may be shaped to direct charged particles 306 generated by a charged particle source 350. The resulting electric field 305 between charged particle source 350 and conductive electrode 310 may be substantially linear with relatively small perturbations in the electric field 305 due to the shape of the structure 330 and the conductive electrode 310.
Because the electric field 305 shown in FIG. 3 is substantially linear, the electric field 305 generally provides a vertical force toward conductive electrode 310 such that the electric field 305 directs the charged particles 306 in a direction vertical from the starting location of the charged particle 306. As a result of the vertical force generated by the substantially linear electric field 305, the charged particles 306 that that are not directly lined up with the aperture 335 of the structure 330 will be directed to locations other than the desired location. A conventional charged particle guide 300 may additionally include introducing airflow through the conventional charged particle guide 300 to further assist the direction of the charged particles 306 toward the aperture 335 in addition to influence from the electric field 305. However, even with the airflow, a substantial quantity of charged particles 306 may not be directed properly to aperture 335 and “die” (i.e., are neutralized) upon contact with the surface of the charged particle guide 300.
Because the electric field 305 between conductive electrode 310 and charged particle source 350 may, at times, adversely affect the efficiency of charged particles 306 entering the aperture 335, another example of a conventional method for directing charged particles 306 may include pulsed dynamic focusing (PDF). With PDF, the voltage applied to charged particle source 350 is dynamically switched from a different voltage from the conductive electrode 310 to a voltage that is equal to the conductive electrode 310 at a timed delay after the charged particles 306 are generated by the charged particle source 350. With an equal voltage between the charged particle source 350 and the conductive electrode 310, the electric field 305 may enter into a null state, after which airflow may be the primary force acting on the charged particles 306 to direct the charged particles 306 to the desired location. In other words, the airflow alone may act to direct off-center charged particles 306 to approach the conductive electrode 310, without the electric field 305 applying a force that may adversely affect charged particles 306 that originate off-center from the aperture 335.
The inventors have appreciated that there is a need for different apparatuses and methods for generating electric fields that may be used to control the motion of charged particles, which may be combined with airflow. The different apparatuses and methods may address one or more of the problems encountered by conventional approaches.